Serious question? Serious answer: Something happened.
My guess is that a world wide catastrophe happened which pushed the “existing civilizations” towards extinction. It would explain why we are finding artifacts that are aged way further back than they should.
Imagine what would happen to us if a massive supervolcano blew up, blocking out the sun. Or if a comet or asteroid hit us.
Sure there would be many who lived for the remainder of their lives. But resources would dry up and it would take years to get back on track. Two or three generations hence would be moving back towards hunter gatherers because they would lack the technology to move the other way.
My guess is that this has happened over, and over, and over again.
Science Fiction is full of this stuff—mostly lifted from mythology and ancient stories.
Or did you not want an answer? :-)